xfrm: validate new SA's prefixlen using SA family when sel.family is unset

This expands the validation introduced in commit 07bf7908 ("xfrm:
Validate address prefix lengths in the xfrm selector.")

syzbot created an SA with
    usersa.sel.family = AF_UNSPEC
    usersa.sel.prefixlen_s = 128
    usersa.family = AF_INET

Because of the AF_UNSPEC selector, verify_newsa_info doesn't put
limits on prefixlen_{s,d}. But then copy_from_user_state sets
x->sel.family to usersa.family (AF_INET). Do the same conversion in
verify_newsa_info before validating prefixlen_{s,d}, since that's how
prefixlen is going to be used later on.

Reported-by:  <syzbot+cc39f136925517aed571@syzkaller.appspotmail.com>
Fixes: 1da177e4 ("Linux-2.6.12-rc2")
Signed-off-by: Sabrina Dubroca <sd@queasysnail.net>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>